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Microsoft Corp. is working with the OPC Foundation to enable almost any IIoT scenario using interoperability between the millions of OPC UA-compliant applications and equipment. Microsoft will enable IIoT users to connect manufacturing equipment and software with extended support of OPC UA's open-source software stack. Microsoft’s extended support for OPC UA spans its IoT offerings from local connectivity with Windows devices to cloud connectivity via Microsoft Azure. Integration with Azure IoT allows customers to easily send OPC UA data to the Azure cloud, as well as command and control OPC UA devices remotely from the Azure cloud.
Pepperl+Fuchs broke ground June 2 on its new U.S. distribution and engineering center in Katy, Texas, near…

While routine build-up and clogging in process applications is annoying, process scaling is more persistent, causes unplanned downtime and increases costs in many industries. It's typically composed of calcium carbonate, wax, grease or similar…

Just as you can't have too many friends, level measurement applications can't get enough ease of use, reliability and safety. Thanks to increasing process industry needs and user demand for them, these are the three unifying priorities that drove…

Our panel of experts explore best practices for dealing with buildup on chemical seals of a differential pressure flowmeter.

Q: We have a venturi flowmeter with 3-in. flanged pressure taps and flanged chemical seals with capillary tubes in a 44-in. vertical pipe (Figure 1). The fluid is hydrocarbon (polymeric) gas. The problem is that the chemical seal diaphragms often…

A Control Design reader writes: We have several temperature, pressure and flow sensors on a new medical-device cleaning skid that we are developing. These instruments are connected to a PLC as 4-20 mA inputs, and there is also a 4-20 mA output used to control a pump motor speed. A recent failure of a flow sensor brought the process skid instrumentation to my company's quality manager's attention. He asked how we know that the temperatures, pressure and flow are accurate, and how do we know that we are cleaning properly.
I've been tasked to write a procedure for troubleshooting, calibrating and testing the 4-20 mA instruments on the skid. I can probably stumble through this, but what are some best practices for troubleshooting the analog…

Control's Monthly Resource Guide brings you the latest online resources on a variety of process control topics.

MULTI-PART WEIGHING
This four-section webpage by Omega Engineering covers "Weighing Applications," "Weighing System Design," "Installation and Calibration" and "Specialized Installations." The whole group is located at…

FOPDT model has demonstrated utility for controller tuning rules, for structuring decouplers and feedforward control algorithms.

A first-order plus deadtime (FOPDT) model is a simple approximation of the dynamic response (the transient or time-response) of a process variable to an influence. It’s also called first-order lag plus deadtime (FOLPDT), or “deadtime” may be…

'To the degree networks and standards can provide easy, consistent and seamless access to device-resident controls, the vision of truly distributed control may finally dawn upon us.'

Every compressor in the facility went down at once that day, when a PLC redundancy switchover didn’t transfer in time. The engineers didn’t know that each P453 remote I/O processor had a dip-switch-selectable timeout setting—if it didn’t…

A Control Design reader writes: We have several temperature, pressure and flow sensors on a new medical-device cleaning skid that we are developing. These instruments are connected to a PLC as 4-20 mA inputs, and there is also a 4-20 mA output used to control a pump motor speed. A recent failure of a flow sensor brought the process skid instrumentation to my company's quality manager's attention. He asked how we know that the temperatures, pressure and flow are accurate, and how do we know that we are cleaning properly.
I've been tasked to write a procedure for troubleshooting, calibrating and testing the 4-20 mA instruments on the skid. I can probably stumble through this, but what are some best practices for troubleshooting the analog…

Most public utilities deliver basic power and water, and God bless them for doing it. However, a few go beyond their essential missions, and seek to further enhance the overall well-being and prosperity of their communities.
One of these is…

Advanced control on fuel and steam can control emissions while saving millions.

It’s no secret that advanced process control (APC) has played a major role in the process industries for many decades. However, while many organizations focus only on major process units, there’s often an untapped opportunity to do more, and…

Our readers tell us who they think delivers the best technology in our industry

There's knowledge in our readers. That's why every year we ask our loyal audience to help us determine which automation service providers offer the best products and brands they can rely on to get the best combination of performance, ease of use,…

Just when it seems like today’s sophisticated motors and drives can’t possibly add more efficiencies and capabilities, engineers conjure up new tricks and refinements, followed by end users and system integrators who materialize new settings and…

Control's monthly guide brings you invaluable industry information to stay up to date on the latest trends and developments.

THE CLASSIC VALVE BOOK
The fourth edition of the 295-page classic text, "Control Valve Handbook," by Fisher Controls International and Emerson Process Management reports this classic text has been a primary reference since its first printing in1965.

Control's monthly guide brings you invaluable industry information to stay up to date on the latest trends and developments.

THE CLASSIC VALVE BOOK
The fourth edition of the 295-page classic text, "Control Valve Handbook," by Fisher Controls International and Emerson Process Management reports this classic text has been a primary reference since its first printing in1965.

Just when it seems like today’s sophisticated motors and drives can’t possibly add more efficiencies and capabilities, engineers conjure up new tricks and refinements, followed by end users and system integrators who materialize new settings and…

Actionable information is essential for the effectiveness of the 'loop,' as it is for closed-loop control and open-loop alarms.

When I sat down in my office this morning, I was greeted by the operations manager, who pointed out an entry from last night’s operations log: “Did you see? The boiler steam vent is in manual because the pressure reading whacked out and made the…

Actionable information is essential for the effectiveness of the 'loop,' as it is for closed-loop control and open-loop alarms.

When I sat down in my office this morning, I was greeted by the operations manager, who pointed out an entry from last night’s operations log: “Did you see? The boiler steam vent is in manual because the pressure reading whacked out and made the…

Actionable information is essential for the effectiveness of the 'loop,' as it is for closed-loop control and open-loop alarms.

When I sat down in my office this morning, I was greeted by the operations manager, who pointed out an entry from last night’s operations log: “Did you see? The boiler steam vent is in manual because the pressure reading whacked out and made the…

Our readers tell us who they think delivers the best technology in our industry

There's knowledge in our readers. That's why every year we ask our loyal audience to help us determine which automation service providers offer the best products and brands they can rely on to get the best combination of performance, ease of use,…

Visualizing and contextualizing safety data can have a positive impact on productivity and compliance.

"Information is one thing, but safety information is quite another," says George Schuster. He should know. He’s charged with business development for the Global Safety Team of Rockwell Automation and has seen many plants and factories begin to…

How to deal with the constraints of putting access points in hazardous areas.

In an ideal world, it wouldn’t be necessary to worry about explosive atmospheres. Unfortunately, plant environments are far from ideal, with most process plants being about 80% Class I, Div. 2 (Zone 2) and another 10% Class I Div. 1 (Zone 1), so…

Key is to learn as much as possible to make digital technologies safe, secure and successful in process settings.

As usual, and I’m sure this is true for many of you, this year’s autumn season has been a frantic dash of work, deadlines, sleep deprivation, indigestion and suspiciously arthritic stiffness. In my case, this means covering all sides of the…

With today’s network technology, even Ethernet and wireless are almost always fast enough.

As automation professionals, one issue we have about control loops is ensuring we're able to support real-time control. Historically, when Ethernet was 10 MB/s and there were multiple drops on a single port, collisions were a significant concern and…

With today’s network technology, even Ethernet and wireless are almost always fast enough.

As automation professionals, one issue we have about control loops is ensuring we're able to support real-time control. Historically, when Ethernet was 10 MB/s and there were multiple drops on a single port, collisions were a significant concern and…

Because they're the crossroads, nexus and Grand Central Station for sensor signals and data arriving and requests and actuation instructions departing, it's a big help that I/O and terminal blocks are more flexible and capable than ever.

Compact, Remote I/O for Zone 2/Div 2
LB System remote I/O has more power in less space with high-performance, compact modules plugged into a backplane. Energy-saving power management and low-power dissipation allow maximum packing density. With the…

Be just 33 seconds more productive each hour, and the savings mount quickly.

Rockwell Automation previewed its new, forward-looking App Platform for mobility at this week’s TechED conference in Orlando. While focused first on redefining smartphone use on the plant floor – with the aim of making individual workers more…

Control’s latest State of Technology report delves into the many aspects of power systems of interest in process plants.

From sourcing and ensuring uninterrupted flows of clean electricity to monitoring, regulating and metering generation and consumption throughout a facility, process automation professionals who want a smooth-running, efficient and reliable facility are well advised to seek and control power.
This anthology of recent articles delves into the many aspects of power systems of interest in process plants. Remote and wireless systems are speeding development and standardization of energy harvesting and improved battery systems.
Line-powered supplies that used to only transform electricity and maybe offer surge protection are now monitoring and measuring current and voltage profiles, implementing alerts and alarms, and sending data up to…

Greg: Here we take advantage of the chance to talk to Bill Thomas, who provides a great lesson of how to succeed in advancing capabilities and opportunities in his career and the control systems for which he was responsible. His career and the…

Control's monthly guide brings you invaluable industry information to stay up to date on the latest trends and developments.

AT THEIR BEST, ALL THE TIME
ExperTune’s white paper, “How to Improve Performance of Process Control Assets,” by George Buckbee, P.E., describes the practice of real-time asset performance management, shows how to get assets to perform at their best all the time, and demonstrates the value of integrating these tools for a performance picture that delivers financial results. It’s available here.
ISA / www.isa.org
ANALYZING, PREDICTING PROBLEMS
This 57-minute video, “Avoid Equipment Surprises: Predictive Diagnostics for Oil & Gas,” is presented by Joe Dupree, GE Automation and Controls, and users from NiSource Gas and Alyeska. They demonstrate condition-based monitoring, predictive analytics and other tools for evaluating…

Steve Christian remembers the days—not too long ago—when production schedules were printed on sheets of paper and distributed to operators at his plant, who would manually twist valve handles to send the Ragu sauce tomato slurry from one kettle…

Advanced control on fuel and steam can control emissions while saving millions.

It’s no secret that advanced process control (APC) has played a major role in the process industries for many decades. However, while many organizations focus only on major process units, there’s often an untapped opportunity to do more, and…

Find and slay the dragons lurking in the typical safety instrumented system.

Cybersecurity is a growing concern in the process industries, and a number of good articles have been written about it for industrial control systems (ICS)—many full of doom and gloom. Here, we will divide the ICS into two parts: safety…

Big data can only help users if they understand what it is, how it can affect their controls and processes, and how they can use it to optimize operations.

Similar to any new technology emerging on the process control front, big data can only help users make better decisions if they understand what it is, how it can affect their controls and processes, and how they can use it to optimize operations.

Our readers tell us who they think delivers the best technology in our industry

There's knowledge in our readers. That's why every year we ask our loyal audience to help us determine which automation service providers offer the best products and brands they can rely on to get the best combination of performance, ease of use,…

Under the hood of override control: Part 2

Users can accomplish forced initialization under adverse conditions, output tracking, and other methods of implementation, including selection based on error and override control using Foundation Fieldbus

ABNORMAL operation in adverse conditions is just a nice way of saying when you-know-what hits the fan. Override control usually refers to applications where one process variable controls the process in normal operation, and during abnormal operation another variable assumes control to prevent encroachment on safety, process or equipment limits. But what if something even worse crops up?

A key element of override control, or selector control, is a hardware or software-based selector function, which selects the lower or higher of two or more controller outputs for passage to the final actuator. "Under the Hood of Override Control: Part 1" [CONTROL Dec. '05] reported that the techniques for preventing reset windup of non-selected controllers will perform comparably to reset feedback if the dynamics of the various loops aren’t too dissimilar. Part 2 covers forced initialization under adverse conditions, output tracking, and other methods of implementation, including selection based on error and override control using Foundation Fieldbus.

Forced InitializationThe behavior of the forced initialization method under adverse, but realistic, circumstances (See Figure 1 below and Sidebar Story at the end of this article) was obtained from simulation of a process in which there was a considerable difference in response of the two process variables to movement of the valve. In addition to dead time, Figure 1’s PV#1 exhibits a very slow response to control action, whereas PV#2 responds quite rapidly, both to control action and to disturbances. The simulation shows what can happen under similar, adverse circumstances. Under milder circumstances, such as a slow load disturbance on the faster loop, these conditions would probably not be observed. Even under milder conditions, however, noisy systems can adversely affect the logic that determines the selected and non-selected controllers, and cause unpredictable results.

The discussion above assumes the use of position mode PID algorithms. Some manufacturers use velocity mode algorithms. The incremental change (Δm) of each controller is added to a register value representing the previously desired output position of that controller. The selector is effectively operating between desired positions, not incremental changes. Logic within the selector adjusts the output of the non-selected controller to be equal to the output of the selected controller, plus the gain-times-error of the non-selected loop. In essence, this method should have the same behavior as the forced initialization method using position mode algorithms.

Output TrackingSome systems adjust the output of the non-selected controller to be the same as that of the selected controller, then force the initialization of the controller to this adjusted value. On the next calculation cycle, each controller adjusts its output value by the required increment of change. Since the outputs were previously forced to equality, the selector device then selects between the incremental changes, rather than the desired positions.

The following scenario describes the problem of responding to incremental changes, instead of desired positions. Suppose the selector device is choosing the lower of the incremental changes of the normal controller (controlling a slow loop) and the abnormal controller (controlling a fast loop). At set-point, the normal controller issues zero-valued incremental changes, while the other controller, with PV below set point, issues positive-valued incremental changes. The normal controller’s output will be selected, resulting in no change to the valve.

Should a disturbance cause the limiting controller to issue a negative-valued incremental change, it will be selected because it’s algebraically lower than the zero-valued incremental change required by the other controller. This will move the valve to a lower position. Suppose the disturbance then disappears before the normal PV has had time to react. The limiting controller will again issue positive-valued incremental changes. The algebraically lower zero-valued incremental change from the normal controller will be selected, resulting in no movement of the valve, even though its position is lower than that required to maintain the normal PV at set-point. This will eventually result in a disturbance to the normal PV, which must experience a deviation from set-point before the valve position can be restored. Had the reset feedback technique been utilized, the correct valve position would have been restored as soon as the disturbance disappeared.

Other Implementation MethodsOther implementers have suggested basing the selection criterion on the loop errors, rather than the loop outputs. Depending on the application, the lowest (algebraic) or highest of the error signals would then be passed to a common PID algorithm (See Figure 2 below). This has the advantage of never having a bump in the controller output caused by switchover from one error signal to another. The disadvantage, however, is that there’s one set of tuning parameters that must apply to all affected process variables, even though their response may be considerably different.

I’m familiar with a custom implementation that circumvented this problem. Separate logic determined which error signal was selected, and obtained a unique set of tuning parameters from a table for that particular process variable. In essence, this was a scheduled tuning approach.

FIGURE 2: OVERRIDE WITH FOUNDATION

Forced initialization method using velocity mode PID algorithms.

Overriding with Foundation FieldbusFoundation Fieldbus is a standard that permits distribution of PID and other types of function blocks into field devices (See Figure 2). The standard defines a control selector (CS) function block. Some manufacturers may not support the CS block, but it supports an inter-block communication, called the “back-calculation procedure,” even when blocks are in separate devices. The CS block provides up to three inputs from other control class blocks, such as PID. Non-configured inputs are ignored. The block also provides a separate BKCAL_OUT signal for each of the inputs. In the automatic mode, the lower (or higher) of the selected input signals is passed to the output. The status returned to the non-selected PIDs via the back-calculation link is “not selected.” Also, the value is the CS output value, which is equivalent to the selected controller’s output. The non-selected controller’s output is then made equal to this value.

In short, the behavior of this system appears to be similar to the output tracking method described previously, so it may be subject to the same potential problems if the dynamics between the controller output and the two process variables is greatly different, that is, if one PV responds quite rapidly and the other quite slowly.

Tracking Forced Initialization in Adverse Conditions

THE FOLLOWING is a detailed analysis of the possible behavior of the forced initialization implementation method under adverse, realistic circumstances. It was obtained by simulating a process that had a considerable difference in response of the two process variables to movement of the valve. (CLICK HERE to see an enlarged version of this chart.)

A sudden disturbance causes PV #2 to rise and the output of its controller to fall below the output of Controller #1. Thus Controller #2 is now the selected controller.

Controller #2 output continues to fall to correct the rise of PV #2. The PV of controller #1, being a slower loop with deadtime is still on set point, so the Controller #1 output tracks Controller #2 output, but with one sample time delay.

At some point, due to the corrective action to PV #2, its controller output reverses and become greater than Controller #1 output. Controller #1 is now the selected controller (for one scan cycle).

Since Controller #2 is now the non-selected controller, its output will be forced to Controller #1 output plus gain-times-error of loop #2. In the example, if PV #2 is above its limit, the error will be negative. Thus, the output of Controller #2 will make a significant decrease, again overriding Controller #1 and becoming the selected controller.

Controller #1 output tracks (delayed by one sample period) Controller #2 output, plus gain-times-error of loop #1. In the example, the error of loop #1 is still small, so its gain-times-error is essentially zero.

The effect of a significant decrease in Controller #2 output causes PV #2 to change fairly rapidly. This in turn causes an increase in Controller #2 output, so that it becomes greater than Controller #1. Controller #1 is once again the selected controller.

Controller #2 output tracks Controller #1 output, plus Gain times Error in loop #2. If by now, PV #2 is below its limit, then the error is positive, so that Controller output #2 will make a jump upward, away from Controller output #1.

The valve is now significantly depressed from the condition required by either PV #1 or by PV #2. It causes a major upset to PV #1 which due to the slow response of loop #1 can be corrected only slowly.

When Controller output #1 crosses over the value for Controller output #2, then Controller output #2 is selected. Controller output #1 now makes a jump upward, since PV #1 is below its set point yielding a positive error.

When equilibrium is retained, unless the initial disturbance to PV #2 is removed, PV #2 will be at it limit value and PV #1 below its set point. Controller #2 will be the selected controller.

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